1. Field of the Invention
The present invention relates to stimulation of a cranial nerve by electrical pulsed signals and, in particular, to the manufacture of a system for electrical stimulation of the vagus nerve and the use of an electrical pulse generator for such a system applicable to the vagus nerve for improving control of cerebellar tremor, particularly, cerebellar tremor associated with multiple sclerosis.
2. Description of Related Art
Patients suffering from multiple sclerosis (MS) show a broad variety of symptoms. Patients diagnosed with MS, e.g., according to the Poser criteria, typically show postural and intentional non-persistent or persistent cerebellar tremor (CT).
Conventionally, patients suffering from MS are treated pharmacologically with drugs or biologically active compounds such as antibiotics or polypeptides. Exemplary pharmaceutical treatments include Interferon β-1b and β-1a. Moreover, several cycles of immunoglobulins may also be administered. Anticonvulsants, e.g., primidone, may alleviate CT by controlling nerve impulses in the brain. In the case of acute relapses corticosteroids are applied.
However, pharmacological treatment may not only result in undesirable side effects but also often proves not sufficiently successful.
Direct electrical stimulation of cranial nerves affecting a variety of brain areas has been successfully used to treat patients for a variety of diseases including epilepsy/epileptic seizures, eating disorders, and depression. Neurocybernetic prostheses as described in the U.S. Pat. No. 4,702,254, has been proven successful in the controlling of epileptic seizures by applying pulsed electrical signals to the vagus nerve.
However, the vagus nerve comprises some 100,000 fibers (axons) of different sizes, generally classified as A, B and C fibers, where A and B fibers are myelinated in contrast to C fibers. Among these fibers are axons conducting signals to the brain (visceral afferents) and nerve fibers conducting signal in the opposite direction (efferents). A, B, and C fibers differ, e.g., in electrical stimulation thresholds and electrical conduction velocities. Individual nerve fibers (whether A, B, or C fibers) generally conduct signals in only one direction. Electrical stimulation of a cranial nerve trunk such as the left or right vagus nerve branches typically generates action potentials both to and from the brain among different (A, B and C) types of nerve fibers. It has been suggested that generally the effects of vagus nerve stimulation are mediated via the nucleus tractus solitarius, the main site of visceral afferent complex termination in the brain, projecting bilaterally to the cerebellum.
Despite the recognition that cranial nerve stimulation may be an appropriate treatment for several movement disorders, e.g., epileptic seizures, the fact that detailed neural pathways for many (if not all) cranial nerves remain unknown makes predictions of efficacy for any given condition or disorder impossible. Moreover, even if such pathways were known, the precise stimulation parameters that would energize particular pathways that affect a particular disorder of interest cannot readily be predicted.
Consequently, electrical stimulation discriminating of a specific kind of nerve fibers, and for particular neural pathways, i.e. neurocybernetic spectral discrimination, represents a tremendous challenge.